This invention relates generally to gel-casting and more particularly to gel-casting of well dispersed aqueous slurry mixtures of powders for WC—Co cemented carbides with high solids loading.
Cemented carbide is a class of materials used in applications where, e.g., high wear resistance is desired. Products of cemented carbide includes inserts for turning, milling or hole making, as well as solid carbide drills, cutting blades and dies used for, e.g., wire drawing. The geometry of the mentioned products can be more or less complex. Different methods exist for forming cemented carbide powders into complex shapes. The methods include processing steps where the powder is by some means transformed to a powder compact, followed by a heat treatment during which the compact forms a solid cemented carbide body. Machining of a solid cemented carbide body to desired shape and dimension is time consuming and expensive and does not always offer enough flexibility. Kingery et al U.S. Pat. No. 3,351,688 discloses a method wherein a refractory powder is mixed with a paraffin binder at a temperature where the binder is liquid, and the resulting mixture is cast into a mold of the desired shape. The viscosities of these mixtures are often such that significant pressure is needed in order to fill the cavity of the mold. The molds are therefore often made of a wear resistant material which makes them time consuming and expensive to manufacture. It would be of great interest if a cemented carbide mixture could be made with such viscosity that the mixture could be cast in a mold made of a material cheap to manufacture. One method of forming complex shapes of cemented carbide powder is gel-casting. The process involves the transition of a system from a liquid suspension into a solid, where the liquid has a viscosity suitable for casting at low applied pressures.
Different organic systems for gel-casting exist, using, e.g., starch gelling, denaturing of proteins or polymerization, Sigmund, Bell and Bergström, “Novel Powder-Processing Methods for Advanced ceramics”, J. Am. Ceram. Soc., 83, 1557, 2,000.
In the case of polymerization, the gel-casting process comprises adding to a powder slurry a polymerizing system, and by subjecting the resulting mixture to, e.g., an increase in temperature, such that the components of the polymerizing system undergo a transition to a solid gel.
Golibersuch, U.S. Pat. No. 2,698,232, discloses casting of thixotropic slurries of cemented carbide and polymerizable agents in organic solvent. The solidification or the hardening of the slurries is controlled by the formation of a gel of the soluble polymerizable agents, e.g., unsaturated polyesters and vinyl derivatives. The polymerization is initiated by the addition of a catalyst and increasing temperature. Golibersuch discloses casting mixtures containing 38-41.5% by volume of cemented carbide powder.
In U.S. Pat. No. 4,894,194, Janney describes a method for gel-casting ceramic powders in organic solvent. In U.S. Pat. Nos. 5,028,362 and 5,145,908, Janney and Omatete disclose the gel-casting of ceramic particles in aqueous solvent, using as the polymerizing vehicle at least one monofunctional monomer and at least one difunctional monomer; the functional groups of these monomers being selected from vinyl or allyl groups. In U.S. Pat. No. 6,066,279 Walls et al disclose a method for forming a gel-cast body of inorganic powder, specifically ceramic powders, without the use of a cross-linking agent, in an aqueous solution. In U.S. Pat. No. 6,228,299, Janney and Walls disclose the use of a plasticizer for improved drying behavior and machinability when gel-casting an inorganic powder using a monomer system.
A critical parameter in gel-casting is the solids loading. A higher solids loading means less segregation of powder in the slurry; a higher density and strength of the green body; less risk of drying cracks when the part is dried; and less shrinkage, thus less form and dimension distortions during sintering. A solids loading of at least 50% by volume is desired, Janney et al., “Development of Low-Toxicity Gelcasting Systems, J. Am. Ceram. Soc., 81, 581, 1998. Optimal gel-casting systems are characterized by a high solids loading and a low viscosity of the slurry. Since the low viscosity is necessary to make the mixture easy to handle and to cast into a mold before gelling, the dispersion of the slurry is a critical issue. For ceramic applications, several dispersants are commercially available and their use is relatively straight-forward.
In the case of WC—Co cemented carbide powders, Bergström and Laarz, US 2002/010219 describe how the surface chemistry of the WC and Co particles makes cemented carbide powders difficult to disperse in polar solvents. In aqueous solution, the WC particles will be covered by an acidic surface oxide, WO3, whereas CoO at the Co-particle surfaces is alkaline. They disclose the use of a cationic polyelectrolyte, polyethyleneimine (PEI), as a dispersant for the fabrication of well dispersed aqueous or ethanol based slurries of cemented carbide powders, which gives a significant decrease of the slurry viscosity as compared to formerly known techniques.
Compared to ethanol based slurries, aqueous slurries present the advantages of being easier to handle since there is no risk of explosions, as well as being environmentally harmless.
However, as the polymerizing system, as well as dispersed slurries of WC—Co cemented carbide powder, consist of several different components, which can interact with each other in different ways, this can lead to unwanted mixture characteristics as well as an obstruction of the gelling process or an unsatisfying gel quality. When applied to aqueous slurries of WC—Co cemented carbide powders, the polymerizing systems suggested by the above mentioned documents either fail to gel or give gels of insufficient mechanical strength. Premature gelling, making the process very difficult to control, is another problem related to such interactions.